The invention relates generally to doors for permitting access to an enclosed space and, more specifically, to a door pivoted about a horizontal axis and associated mechanism which provides access to the interior of a chamber.
Since doors are simple structures with commonly accepted features and functions, they are oftentimes taken for granted. At the same time, however, they are highly sophisticated and adapted to their individual purpose. One such set of sophisticated constraints and requirements relates to doors for vacuum processing of produce. In such equipment, it is necessary that a door open and move away completely from the end of a vacuum chamber providing substantially complete and unhindered access into the interior of the chamber. Such doors are typically relatively massive inasmuch as they must withstand reduced internal pressure on the order of 5 millimeters of mercury. The necessary movement of such a door therefore presents certain challenges. Furthermore, it is preferable that such doors, while in their open positions, or moving between open and closed positions, do not interfere with adjacent structures such as loading docks, roofs and the like.
One such conventional vacuum cooler door is a vertical gate type wherein a door having an area and configuration comparable to the cross section of the vacuum chamber is disposed upon and translatable within a pair of parallel, vertically disposed tracks having a length approximately twice the height of the vacuum chamber. Such a door is especially suitable to vacuum chambers having a square cross section since it permits ready access to the entire end of the vacuum chamber when moved vertically upward a distance equal to its height. A major drawback of this design, however, is that it requires significant overhead clearance equal to approximately twice the height of the vacuum chamber. This requirement may necessitate modification to building ceilings in order to accommodate the significant vertical height of such a mechanism. Furthermore, this configuration does not lend itself to portable vacuum chambers since, again, the vertical door tracks will extend above the vacuum chamber a distance approximately equal to its own height.
Another design which minimizes certain problems of the above-described gate type door, comprehends a door which is pivotally secured to and supported along its horizontal midpoint axis by the ends of a pair of first class levers having their center pivots secured to the sides of the vacuum chamber. A pair of linear actuators such as hydraulic cylinders are pivotally secured to the other ends of the lever arms. The upper, central portion of the door is constrained for limited movement along a generally horizontal axis. So constrained, the action of the lever arms lifts the door upwardly and away from the end of the vacuum chamber thereby providing access thereinto. Generally speaking, a vacuum cooler having a door of this design is more readily portable inasmuch as difficulties attendant the vertical height of the gate door trucks are eliminated. However, the lever arm and operator mechanism disposed on each side of the chamber effects a width limitation on the chamber if the apparatus is to meet federal and state vehicle width restrictions.
The utilization of a pair of hydraulic cylinders may also pose difficulties. First of all, if the cylinders are operating off equal hydraulic pressure, any binding or uneven load on the door will result in uneven travel thereof. Such skewing may compound binding of the pivots and bearings and damage to the door opening mechanism may result. A somewhat related problem can develop if any intermittent binding or external force excites the door into simple harmonic motion at its resonant frequency. In a dual cylinder configuration, such resonant motion of the door may induce a sympathetically alternating flow of hydraulic fluid between the two cylinders. Since there is negligible hydraulic damping, or damping from any other source for that matter, the most expeditious fashion in which to eliminate such resonant harmonic motion is to terminate the opening or closing cycle until such motion dies away and then continue the opening or closing cycle. Such interruptions are, of course, annoying and time consuming but, the alternative, of not terminating motion of the door, is even less desirable since it may cause damage to the door and operator mechanism.
Finally, though motion of the upper portion of the door is constrained along a horizontal axis between two finite limits, motion between such limits is unrestricted and usually occurs suddenly. Though such sudden motion may well be anticipated by the operator of the door, it may surprise others and deleteriously affect the life of the door opening mechanism and supporting chamber structure.
From the foregoing, it is apparent that improvements in the art and design of vacuum cooler doors and door opening assemblies are both possible and desirable.